Virus

Viruses are genetic elements enclosed in protein and are not considered to be organisms, as they
cannot reproduce independently. Because of their disease producing potential, viruses are
important biological entities. The virus particles produce the important disease influenza. Other
viruses cause AIDS, polio, flu, and some can lead to cancer.

Viral diversity
A sample of the
extensive diversity and
small size viruses is
depicted. At the scale
these viruses are
shown, a human hair
would be nearly 8
meters thick.

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The Discovery of Viruses

Viruses possess only a portion of the properties of organisms. Viruses are literally ―parasitic‖
chemicals, segments of DNA or RNA wrapped in a protein coat. They cannot reproduce on their
own, and for this reason they are not considered alive by biologists. They can, however,
reproduce within cells, often with disastrous results to the host organism. Earlier theories that
viruses represent a kind of halfway point between life and nonlife have largely been abandoned.
Instead, viruses are now viewed as detached fragments of the genomes of organisms due to the
high degree of similarity found among some viral and eukaryotic genes.

Viruses vary greatly in appearance and size. The smallest are only about 17 nanometers in
diameter, and the largest are up to 1000 nanometers (1 micrometer) in their greatest dimension.
The largest viruses are barely visible with a light microscope, but viral morphology is best
revealed using the electron microscope. Viruses are so small that they are comparable to
molecules in size; a hydrogen atom is about 0.1 nanometer in diameter, and a large protein
molecule is several hundred nanometers in its greatest dimension.

Biologists first began to suspect the existence of viruses near the end of the nineteenth century.
European scientists attempting to isolate the infectious agent responsible for hoof-and-mouth
disease in cattle concluded that it was smaller than a bacterium. Investigating the agent further,
the scientists found that it could not multiply in solution—it could only reproduce itself within
living host cells that it infected. The infecting agents were called viruses.

The true nature of viruses was discovered in 1933, when the biologist Wendell Stanley prepared
an extract of a plant virus called tobacco mosaic virus (TMV) and attempted to purify it. To his
great surprise, the purified TMV preparation precipitated in the form of crystals. This was
surprising because precipitation is something that only chemicals do the TMV virus was acting
like a chemical off the shelf rather than an organism. Stanley concluded that TMV is best
regarded as just that chemical matter rather than a living organism.

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The Nature of Viruses
Viral Structure
All viruses have the same basic structure: a core of nucleic acid surrounded by protein.
Individual viruses contain only a single type of nucleic acid, either DNA or RNA. The DNA or
RNA genome may be linear or circular, and single-stranded or double-stranded.

The structure of a bacterial, plant, and animal virus. (a) Bacterial viruses, called
bacteriophages, often have a complex structure. (b) TMV infects plants and consists of
2130 identical protein molecules (purple) that form a cylindrical coat around the single
strand of RNA (green). The RNA backbone determines the shape of the virus and is
protected by the identical protein molecules packed tightly around it. (c) In the human
immunodeficiency virus (HIV), the RNA core is held within a capsid that is encased by a
protein envelope.
Viruses are frequently classified by the nature of their genomes. RNA-based viruses are known
as retroviruses. Nearly all viruses form a protein sheath, or capsid, around their nucleic acid core.
The capsid is composed of one to a few different protein molecules repeated many times. In
some viruses, specialized enzymes are stored within the capsid. Many animal viruses form an
envelope around the capsid rich in proteins, lipids, and glycoprotein molecules. While some of
the material of the envelope is derived from the host cell’s membrane, the envelope does contain
proteins derived from viral genes as well.

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Viral Replication
Viruses can reproduce only when they enter cells and utilize the cellular machinery of their
hosts. Viruses code their genes on a single type of nucleic acid, either DNA or RNA, but viruses
lack ribosomes and the enzymes necessary for protein synthesis. Viruses are able to reproduce
because their genes are translated into proteins by the cell’s genetic machinery. These proteins
lead to the production of more viruses.

Viral Shape
Most viruses have an overall structure that is either helical or isometric. Helical viruses, such as
the tobacco mosaic virus, have a rodlike or threadlike appearance. Isometric viruses have a
roughly spherical shape whose geometry is revealed only under the highest magnification. The
only structural pattern found so far among isometric viruses is the icosahedron, a structure with
20 equilateral triangular facets, like the adenovirus. Most viruses are icosahedral in basic
structure. The icosahedron is the basic design of the geodesic dome. It is the most efficient
symmetrical arrangement that linear subunits can take to form a shell with maximum internal
capacity.
CLASSIFICATION OF VIRUSES
Viruses are classified according to:
Nucleic acid:
 type: DNA or RNA
 double stranded/single stranded
 single or segmented pieces of nucleic acid
 positive or negative stranded RNA
 complexity of genome
Capsid:
 icosahedral
 helical
Envelope:


naked
enveloped
Size:
 diameter of helical capsid virus
 number of capsomeres in icosahedral capsid.

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ADENOVIRUS
Characteristics
 Belong to family Adenoviridae,
genus Mastadenovirus
 DNA, double stranded, linear,
icosahedral, none enveloped.
 Has protruding fibers
 are its form of attachment
 are toxic to humans
 are the main type specific antigen (has 41 antigenic types)
· Classified into 7 groups based on DNA morphology:
- Group A: no clinical disease in humans
- Group B: acute resp. disease, fever, hemorrhagic cystitis
- Group C: latent infection in tonsils + other lymphatic organs
- Group D: sporadic and epidemic Keratoconjuctivitis + pinkeye
- Group E: acute resp. disease with fever
- Group F and G: gastroenteritis
Replication cycle
· DNA viral replication
· Replication and reassembly takes place in nucleus
· Virus released by lysis of cell (no envelope)
Diseases
· Upper and lower respiratory tract infection pharyngitis, pneumonia (group B, C, E)
· Diarrhea (by enteric strains F, G)
· Conjunctivitis (group D)
· Hemorrhagic cystitis (group B)
· Gastroenteritis (group F and G)
Transmission
· Aerosol droplets (mainly)
· Faecal oral route in enteric strains
· Direct transmission through finger in conjunctivitis (infected animals do not infect humans)

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Pathogenesis
· Infect mucosal epithelium of resp. tract, eyes, and G.I.
· Gives acute (can lead to death) and late infections (occurs mainly in pharynx with low virus
production and no symptoms)
The virus is capable of infecting multiple organ systems; however, most infections are
asymptomatic. Adenovirus is often cultured from the pharynx and stool of asymptomatic
children, and most adults have measurable titers of anti-adenovirus antibodies, implying prior
infection. Adenovirus is known to be oncogenic in rodents but not in humans.
Clinical findings
· Sore throat
· Inflammation of mucosal membrane
· Most resolve spontaneously
· 50% asymptomatic
Treatment
· No antiviral drug therapy

RETROVIRIUS
Retroviridae is a family of enveloped viruses that replicate in a host cell through the process of
reverse transcription. A retrovirus is a single-stranded RNA virus that stores its nucleic acid in
the form of an mRNA genome (including the 5' cap and 3' PolyA tail) and targets a host cell as
an obligate parasite. Once inside the host cell cytoplasm, the virus uses its own reverse
transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern,
thus retro (backwards). This new DNA is then incorporated into the host cell genome by an
integrase enzyme, at which point the retroviral DNA is referred to as a provirus. The host cell
then treats the viral DNA as part of its own genome, translating and transcribing the viral genes
along with the cell's own genes, producing the proteins required to assemble new copies of the
virus. It is difficult to detect the virus until it has infected the host. At that point, the infection
will persist indefinitely.

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Replication cycle
 Retro viral replication
Pathogenesis
 The virus that causes AIDS (acquired
immune deficiency syndrome) is a
retrovirus. It is called HIV for human
immunodeficiency virus.

Transmission
 Cell-to-cell
 Fluids
 Airborne, like the Jaagsiekte sheep retrovirus.
Treatment
Antiretroviral drugs are medications for the treatment of infection by retroviruses, primarily
HIV. Different classes of antiretroviral drugs act on different stages of the HIV life cycle.
Combination of several (typically three or four) antiretroviral drugs is known as highly active
anti-retroviral therapy (HAART).

TUMOR VIRUSES
The tumor viruses can be classified into:
 DNA tumor virus
 RNA tumor virus
Members of six distinct families of animal viruses, called tumor viruses, are capable of directly
causing cancer in either experimental animals or humans. Viruses belonging to five of these
families have DNA genomes and are referred to as DNA tumor viruses. Members of the sixth
family of tumor viruses, the retroviruses, have RNA genomes in virus particles but replicate via
synthesis of a DNA provirus in infected cells. The viruses that cause human cancer include
hepatitis B virus (liver cancer), papillomaviruses (cervical and other anogenital cancers),
Epstein-Barr virus (Burkitt's lymphoma and nasopharyngeal carcinoma), Kaposi's sarcoma-
associated herpesvirus (Kaposi's sarcoma), and human T-cell lymphotropic virus (adult T-cell
leukemia). In addition, HIV is indirectly responsible for the cancers that develop in AIDS
patients as a result of immunodeficiency, and hepatitis C virus (an RNA virus) is an indirect
cause of liver cancers resulting from chronic tissue damage.

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Virus family Human tumors Genome size
(kb)

DNA tumor viruses

Hepatitis B viruses Liver cancer 3

SV40 and None 5

polyomavirus

Papillomaviruses Cervical carcinoma 8

Adenoviruses None 35

Herpesviruses Burkitt's lymphoma, nasopharyngeal carcinoma, 100–200

Kaposi's sarcoma

RNA tumor viruses

Retroviruses Adult T-cell leukemia 9

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